The reaction between NO(g) and O2(g) to produce NO2(g) in a rigid reaction vessel is represented in — States of Matter Chemistry Question

STEPS:

1. Identify the chemical reaction and write the balanced equation: The reaction between nitrogen monoxide gas and oxygen gas to produce nitrogen dioxide gas is represented by the equation:
$$2\ \text{NO}(g) + \text{O}_2(g) \rightarrow 2\ \text{NO}_2(g)$$
2. Examine the stoichiometry of the reaction: Notice the molar ratio of the gaseous reactants to gaseous products:
* On the reactant (left) side, there are $3\text{ moles}$ of gas ($2\text{ moles of NO} + 1\text{ mole of O}_2$).
* On the product (right) side, there are $2\text{ moles}$ of gas ($2\text{ moles of NO}_2$).
* This means that as the reaction proceeds to completion, every time the reaction occurs, three gas molecules are consumed to produce only two gas molecules, leading to a net decrease in the total number of gas molecules in the vessel.
3. Relate the number of gas molecules to pressure using the Ideal Gas Law: The Ideal Gas Law is defined as:
$$PV = nRT$$
The problem specifies that the reaction occurs in a rigid reaction vessel (meaning volume, $V$, is constant) and at a constant temperature ($T$ is constant). Under these conditions, pressure is directly proportional to the number of moles of gas particles ($P \propto n$).
4. Determine the pressure change: Because the total number of gas molecules ($n$) decreases as reactants are converted into products, the frequency of molecular collisions with the walls of the container must also decrease.
5. Conclude the correct option: A lower collision frequency results in a decrease in the pressure inside the vessel because there are fewer molecules of product than reactants, making Option B the correct choice.

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WHY_OTHERS_WRONG:

• Option A is incorrect: Although a single molecule of product ($\text{NO}_2$, $46\text{ g/mol}$) has a greater mass than a single molecule of reactant ($\text{NO}$, $30\text{ g/mol}$ or $\text{O}_2$, $32\text{ g/mol}$), gas pressure does not depend on the mass of individual molecules. According to Kinetic Molecular Theory, at a given temperature, heavier molecules simply move slower on average, resulting in the same average kinetic energy and identical pressure contributions per particle. Furthermore, the pressure will decrease, not increase, due to the reduction in particle count.
• Option C is incorrect: While it is true that the heavier product molecules ($\text{NO}_2$) will have a lower average speed than the lighter reactant molecules at a constant temperature, this speed difference is not the cause of the pressure drop. If the number of moles of gas remained constant, a difference in molecular mass/speed would have zero effect on pressure because the greater mass of the slower-moving particles perfectly offsets their lower speed during wall collisions. The pressure decreases solely because there are *fewer* total particles colliding with the walls.
• Option D is incorrect: While the total mass of the system is indeed conserved and does not change (the mass of the products equals the mass of the reactants), this conservation of mass does not prevent the pressure from changing. Pressure is a macroscopic property governed by the quantity of independent gas particles ($n$), which decreases during this reaction, rather than the total mass of those particles.